The present invention relates to piezoelectric/electrostrictive elements, and in particular to piezoelectric/electrostrictive elements employed as actuators utilizing flexural displacement and sensors for detecting fluid properties, sound pressure, minute weights and accelerations, etc., as for example, in microphones or viscosity sensors.
Piezoelectric/electrostrictive film elements are utilized in various types of actuators and sensor devices. The various applications of piezoelectric/electrostrictive film elements include the measuring of various properties of fluids, such as the measuring of the properties of density, concentration and viscosity, etc., as disclosed, for example, in Japanese Patent publication No. 8-201265A. Such elements are conveniently employed as sensors because there is a correlation between the amplification of a piezoelectric/electrostrictive oscillator and the viscosity resistance of a fluid in contact with the oscillator. To quantify this correlation, piezoelectric/electrostrictive elements exploit the principal that the form of oscillation in a mechanical system like the oscillation of an oscillator can be converted to an equivalent circuit in an electrical system. For example, a piezoelectric/electrostrictive film oscillates in a fluid and receives a mechanical resistance based on the viscosity resistance of the fluid. Based on the above-mentioned principle, the oscillator thereby senses the variation of an electrical constant of an equivalent electrical circuit of the piezoelectric/electrostrictive element configuring the oscillator. As a result, it becomes possible to measure various parameters, which include the viscosity, density and concentration of the fluid.
A piezoelectric/electrostrictive film oscillator has the capability to measure fluids in both the liquid and gas phases. Moreover, the above oscillator is capable of not only measuring liquids consisting of a single constituent element (i.e., water, alcohol, or oils, etc.) but may also measure fluids composed of slurries and pastes into which a soluble or insoluble medium is dissolved, mixed or suspended.
Examples of the electrical constants that a piezoelectric/electrostrictive oscillator is capable of detecting include loss factor, phase, resistance, reactance, conductance, susceptance, inductance and capacitance. Particularly preferred electrical constants are phase and loss factors because they have a single maximum or minimum point of variation near the resonance frequency of an equivalent circuit. Consequently, not only can the viscosity of a fluid be measured, but its density and concentration are capable of being quantified as well. For example, the concentration of sulfuric acid in an aqueous solution of sulfuric acid can be measured through the use of the above electrical constants. Furthermore, in addition to the use of electrical constants, the variation in resonance frequency may also be utilized as an index for sensing variations in the form of oscillation insofar as there are no specific problems from the standpoint of precision of measurement and durability.
FIG. 2 illustrates a conventional piezoelectric/electrostrictive film element as disclosed in Japanese Patent publication No. 5-267742A. An auxiliary electrode 8 is formed at a position independent of a lower electrode 4, which is laminated on a ceramic substrate 1 having a thin diaphragm 3 and a thicker portion 2. The fluid to be analyzed is introduced into a hollow portion 10 via through-hole 9. A portion of the auxiliary electrode is positioned beneath a piezoelectric/electrostrictive film 5. As a result of this configuration, it is possible to improve the reliability of the connection of an upper electrode 6 through the continuous formation (i.e., without a break in the connection) of the upper electrode on the face of the auxiliary electrode 8 and the piezoelectric/electrostrictive film 5.
A piezoelectric/electrostrictive film element is also disclosed in Japanese Patent publication No. 6-260694A. As shown in FIG. 2, a piezoelectric/electrostrictive film 5 is positioned on a lower electrode 4 and is of a size that the surrounding portion of the piezoelectric/electrostrictive film 5 extends beyond the electrode 4. As a consequence, it is not necessary to precisely position the lower electrode 4 and the piezoelectric/electrostrictive film 5, and thus short circuits between the upper and the lower electrodes are easily prevented. Additionally, an extending portion 11 of the piezoelectric/electrostrictive film can manifest more than sufficient flexural displacement, generation and oscillation because it is in an incompletely bonded state with the substrate 1 (i.e., the extended portion 11 is not bonded with the substrate due to the purpose of incompletely bonded portions 7A). An xe2x80x9cincompletely bonded statexe2x80x9d means that a portion of the extending portion 11 is either partially bonded to the ceramic substrate or that an unbonded region without any bonded portion is in existance. More specifically, xe2x80x9cincompletely bonded statexe2x80x9d is defined to mean that the peeling (tear-off) strength of the film 5 to the substrate 1 is 0.5 kg/mm2 or less.
With respective to the formation of an unbonded state as described above, there are instances when it is necessary to have a low reactivity between the materials selected for the substrate and the piezoelectric/electrostrictive film. In this regard, it is also possible to form a dummy layer between the piezoelectric/electrostrictive film and the substrate so as to prevent their direct contact. Ideally, the dummy layer is formed by a stamping method, a screen printing method or an ink jet method. The incompletely bonded portion 7A is formed when the dummy layer is subsequently dissolved. For example, the dummy layer is fabricated with combustible/removable materials (i.e., resin materials, etc.,) that are dissolved away to form the incompletely bonded portions 7A when the piezoelectric/electrostrictive film 5 is heat treated. Alternatively, in the case where the piezoelectric/electrostrictive film and the upper electrode are not heat treated, the dummy layer is formed with a resin material to be dissolved in a composition such as water or organic solvents, etc. Accordingly, after the formation of either the piezoelectric/electrostrictive film 5 alone or in conjunction with the upper electrode 6, the incompletely bonded portion 7A is formed by dissolving or removing the dummy layer (i.e., water or organic solvents, etc.).
In the above-described prior art piezoelectric/electrostrictive oscillators, the electrical constants between the individual sensor elements tend to vary in both the initial phase with the subsequent passage of time. In such cases, a bothersome fine-tuning process is required to insure the proper performance of the oscillator. In such prior art piezoelectric/electrostrictive oscillators employed as sensor elements utilizing electrical constants, an incompletely bonded portion 7B, which is in the same incompletely bonded state as the incompletely bonded state 7A of the extending portion 11, is formed between the lower electrode 4 and auxiliary electrode 8. Variations and alterations in the incompletely bonded state of this incompletely bonded portion 7B are the principal cause of changes in the oscillation of the sensor elements, which, in turn, yields alterations in the electrical constants of prior art piezoelectric/electrostrictive oscillators. That is to say that the incompletely bonded state of prior art devices is a drawback because the incompletely bonded state 7B is not reliably replicated. For example, since the thin diaphragm oscillates or is displaced, partial destruction of the bond or microscopic cracking at the portion of 7B is likely to occur when the oscillator is in operation.
The present invention relates a piezoelectric/electrostrictive element comprising successively laminated layers. A lower electrode and an auxiliary electrode having a space formed therebetween are laminated on a ceramic substrate. A bonding layer comprising an insulator is formed on the ceramic substrate between the lower and auxiliary electrodes. A piezoelectric/electrostrictive layer is formed on at least a portion of each of the lower electrode, the bonding layer and the auxiliary electrode. An upper electrode extends over the piezoelectric/electrostrictive layer and contacts the auxiliary electrode. The bonding layer forms a completely bonded portion that comprises portions of the ceramic substrate, the lower electrode, the auxiliary electrode and the piezoelectric/electrostrictive layer.
xe2x80x9cCompletely bondedxe2x80x9d refers to the peeling (tear-off) strength of the piezoelectric/electrostrictive film after the substrate, bonding layer and piezoelectric/electrostrictive film have been integrated into a unit. The peeling (tear-off) strength is 2 kg/mm2 or greater.
The piezoelectric/electrostrictive layer preferably comprises at least one material selected from the group consisting of lead titanate, lead zirconate, lead-magnesium niobate and lead-nickel niobate. The piezoelectric/electrostrictive layer may also be preferably formed from (Bi0.5Na0.5)TiO3 or a material having (Bi0.5Na0.5)TiO3 as its principal constituent. Furthermore, the piezoelectric/electrostrictive layer more preferably comprises (1xe2x88x92x) (Bi0.5Na0.5)3xe2x80x94xKNbO3(0xe2x89xa6xxe2x89xa60.06 in mole proportions) or a material having (1xe2x88x92x)(Bi0.5Na0.5)TiO3xe2x80x94xKNbO3 (0xe2x89xa6xxe2x89xa60.06 in mole proportions) as its principal constituent.
It is preferable that the bonding layer of the piezoelectric/electrostrictive film element be selected from an insulator material having a softening point at or above the heat treating temperature of the piezoelectric/electrostrictive layer.
The piezoelectric/electrostrictive layer can be completely bonded to the ceramic substrate most effectively when it is formed from either (Bi0.5Na0.5)TiO3 or (1xe2x88x92x)(Bi0.5Na0.5)TiO3xe2x80x94xKNbO3 (0xe2x89xa6xxe2x89xa60.06 in mole proportions) or a material having either one of the above as its principal constituent. In both of the above configurations, the completely bonded state is brought about by providing a bonding layer formed on the ceramic substrate between the lower electrode and the auxiliary electrode, both of which are also laminated on the ceramic substrate. The insulator material of the bonding layer is formed of (1xe2x88x92x)(Bi0.5Na0.5)TiO3xe2x80x94xKNbO3 (0.08 less than x less than 0.5 in mole proportions) or a material having (1xc3x97x) (Bi0.5Na0.5)TiO3xe2x80x94xKNbO3 (0.08 less than x less than 0.5 in mole proportions) as its principal constituent. Employing the above bonding layer results in a completely bonded state between portions of the ceramic substrate, the lower electrode, the auxiliary electrode and the piezoelectric/electrostrictive layer.